1. Field of the Invention
The invention relates to a moving target indication radar and, more particularly, a radar of this kind having a high accuracy in the detection of azimuthal positions of moving targets.
2. Prior Art
A radar system, particularly an air traffic control (ATC) radar such as an airport surveillance radar (ASR) and an air route surveillance radar (ARSR) must not only discriminate the returns from moving objects (targets) from undesirable returns from stationary objects such as buildings and hills but also must detect a plurality of targets separately from one another.
The undesired returns include ground clutter attributed to buildings and undulating terrains, sea clutter caused by sea surface, weather clutter caused by rain fall and rain clouds, "angel echo" arising from large flocks of migrating birds, and the like. The conventional MTI (moving target indicator) is well adapted for rejecting ground clutter. A MTI canceller, however, is unable to reject clutter having a speed component such as sea clutter, "angel echo," weather clutter and the like. On the other hand, the MTI canceller rejects, together with the ground clutter, the returns from a target having zero or close-to-zero Doppler speed component such as an aircraft flying tangentially to the radar system.
The Log-CFAR (Logarithmic Amplification and Constant False Alarm Rate) technique, which has been proposed to alleviate these disadvantages, is discussed in detail in a paper entitled "Detection Performance of the Cell Averaging Log/CFAR Receiver" by V. G. Hansen and H. R. Ward, IEEE Transaction of AES-8, p-648, 1972. The Log-CFAR technique, based on the fact that the sea and weather clutter has an amplitude distribution similar to the Rayleigh distribution, employs the combination of a logarithmic amplifier and a CFAR circuit to suppress the clutter components to a level comparable to the noise level inherent to the radar receiver. However, desired target detection is impossible for the Log-CFAR technique when the target returns are not higher in level than the moving clutter.
Another problem involved in the Log-CFAR technique resides in the detection of a plurality of targets located at about the same distance from the radar system which have different Doppler speeds. That is to say, the technique cannot separately detect such targets for every Doppler speed. The application of a Log-CFAR to an ATC radar system is far from being sufficient because that the separate detection of targets is essential to an ATC radar system.
These difficulties involved in the conventional techniques are attributed to the fact that the signal processing for the clutter rejection and target detection are all performed in the time domain. To overcome these difficulties, the signal processing must be performed in the frequency domain. To achieve this, the radar signal must be converted through Fourier transform to various clutter and target components mutually separated in the frequency domain, and the separated components must be processed on a real time basis.
It is the fast Fourier transform that provides the basis for real time processing. An algorithm for the fast Fourier transform was proposed by J. W. Cooley et al in an article "An Algorithm for the Machine Calculation of Complex Fourier Series" Mathematics Computation, Volume 19, No. 90, page 297, April 1965. Moreover, the marked reduction in the manufacturing cost of the circuit to perform the algorithm made possible by the recent progress in LSI technology has induced various proposals for the circuits for the fast Fourier transform.
A typical example of those circuits is described by G. C. O'Leary in a paper "Nonrecursive Digital Filter Using Cascade Fast Fourier Transformers", published in IEEE Transaction on Audio and Electroacoustic, Volume AU-18, No. 2, June 1970.
For a better understanding of the O'Leary's proposal, the nature of a radar reception signal will now be briefly described.
A microwave pulse (a radar scanning pulse) is radiated from an antenna rotating at a constant speed into space and is reflected at stationary objects and targets in the scanned space with the result that the returns are obtained for every azimuth region corresponding to one radar scanning pulse (referred to as a unit azimuth region) in a chain of radar data respectively representative of the objects and targets existing respectively in the range regions (referred to as unit range regions) each corresponding to the width of the scanning pulse. (When the radar reception signal is sampled by a pulse having a repetition period equal to one half of the scanning pulse width, the unit range region is one half of that defined above). Since such a radar data chain is obtained for each scanning pulse the radar data chains are successively obtained, as the scanning pulses are radiated into space at a fixed repetition rate.
The radar data corresponding to each of the unit range regions (a unit region also in the azimuth direction) represents the vector sum of the return energy from stationary objects and targets lying in the unit range region. Since the beam pattern of the transmitting antenna has a width large enough to cover a plurality of unit azimuth regions, each scanning pulse always irradiates a plurality of unit azimuth regions. Therefore, the radar data obtained from adjacent unit azimuth regions show a considerably high correlation between them. The Fourier transform mentioned above is based on this correlation present in the radar data. More particularly, the radar data obtained from the adjacent unit azimuth regions for each unit distance in the range direction are subjected to correlation analysis to detect Doppler frequency components (including zero Dopper speed component) of the returns from stationary objects and targets in the scanned space, for every unit range distance over all the azimuth directions.
Based on the above-outlined nature of radar data, the O'Leary circuit is so designed that the radar data incoming successively from eight unit azimuth regions (located at the same distance in the range direction) are successively subjected to a series operation to produce eight Doppler frequency components (including a zero Doppler speed component). It should be noted here that while the O'Leary paper does not clearly state that the input data is a radar data, the input data treated there is exactly the same as the above-mentioned radar data. This conventional circuit proposed by O'Leary is arranged to produce eight Doppler frequency components, i.e., eight Fourier transform outputs, for the corresponding eight input radar data samples, so that the output appears intermittently at an interval eight times as large as that of the input radar data. In other words, one set of Fourier transform outputs is produced for every eight radar scanning pulses.
Therefore, in a radar system based on the O'Leary circuits, the azimuth direction resolution for the target detection is merely one-eighth of the original resolution (which was high enough to recognize targets on one unit azimuth region basis). The low resolution in the azimuthal direction means that a plurality of targets lying adjacent to each other in the azimuthal direction cannot be separately detected, and that the performance of the ATC radar system is limited accordingly.